The mechanism of Ca2+ regulation of vascular smooth muscle thin filaments by caldesmon and calmodulin

Effect of unphosphorylated smooth muscle myosin on caldesmon-mediated regulation of actin filament velocity

The effect of smooth muscle myosin at different levels of light chain phosphorylation on caldesmon-mediated movement of actin filaments was investigated using an in vitro motility assay. Myosin at different levels of phosphorylation was obtained by mixing different proportions of fully phosphorylated and unphosphorylated myosin in monomeric form, while keeping the total myosin concentration constant. The average velocity of actin filaments containing tropomyosin was 1.20 +/- 0.046 microns s-1 at 30 degrees C with fully phosphorylated myosin. This velocity was not altered when the percentage of unphosphorylated myosin coated on the nitrocellulose surface was increased to 80%; further increases lowered the velocity. When the actin filaments with caldesmon bound at stoichiometric levels were used, filament velocity was unaffected until 50% of the myosin was unphosphorylated, but further increases in the percentage of unphosphorylated myosin induced a decrease in the velocity, and at 95% unphosphorylated myosin, filament movement had ceased. The decreased filament velocity in the presence of caldesmon was also observed when phosphorylated myosin was mixed with myosin rod instead of unphosphorylated myosin, but was not observed when the 38 kDa caldesmon C-terminal fragment, which lacks the myosin-binding domain, was used instead of intact caldesmon. These data indicate that the decreased filament velocity in the presence of caldesmon reflects the mechanical load produced by the tethering of actin to myosin through the interaction of the caldesmon N-terminal domain and the myosin S-2 region. The tethering effect mediated by caldesmon may play a role in smooth muscle contraction when a large number of myosin heads are dephosphorylated, as in force maintenance.

Filamin and gelsolin influence Ca(2+)-sensitivity of smooth muscle thin filaments

Sheep aorta thin filaments were prepared by ultracentrifugation of an ATP-containing extract in the presence of different concentrations of ethanediol. Thin filaments prepared without ethanediol contained small quantities of tropomyosin (0.027 Tm:actin) and caldesmon (0.017 CD:actin) and activated the MgATPase of skeletal myosin independently of Ca2+. Ultracentrifugation in the presence of 10-20% ethanediol resulted in preparation of thin filaments with increased content of tropomyosin (0.17 Tm:actin) and caldesmon (0.04 CD:actin). These thin filaments possessed high Ca(2+)-sensitivity in activation of skeletal muscle myosin ATPase. Besides actin, tropomyosin and caldesmon, thin filaments contained gelsolin and filamin. Gelsolin content (0.007 gelsolin:actin) was independent of the presence of ethanediol. The filamin content decreased from 0.015 to 0.007 mol:mol actin when the ethanediol concentration was increased from 0 to 20%, and was negatively correlated with the Ca2+ sensitivity of thin filaments. In a reconstituted system, pure filamin or gelsolin affected caldesmon regulation of actomyosin ATPase. Gelsolin (0.01:actin) reduced the inhibition of actomyosin ATPase caused by caldesmon and increased the potency of Ca(2+)-calmodulin in reversing this inhibition. Filamin (0.007:actin) also decreased the inhibitory action of caldesmon on actin-activated myosin ATPase and also potentiated the reversal of this inhibition by calmodulin. We conclude that minor components of smooth muscle thin filaments (gelsolin and filamin) significantly modify caldesmon mediated regulation of actomyosin ATPase. We suggest a tropomyosin-mediated mechanism by which filamin or gelsolin could exert similar effects.

Contractile units in stress fibers of fetal human astroglia in tissue culture

The interaction between myosin and F-actin requires the enzyme, myosin light chain kinase (MLCK), as well as Ca(2+)-calmodulin and the calmodulin binding protein, caldesmon, which also binds to F-actin. Using immunofluorescence staining, we have demonstrated that in human fetal astroglia as in mouse astroglia (Abd-El-Basset et al., 1991) the stress fibers contain these contractile elements: F-actin, myosin, tropomyosin and caldesmon. F-actin extends continuously along the stress fibers, whereas myosin, tropomyosin and caldesmon are localized discontinuously in a periodic pattern. In addition, we have demonstrated that fetal human astroglia have the enzyme MLCK and calmodulin. The association of the contractile elements listed above together with calmodulin and MLCK constitutes what may be termed 'contractile units', suggesting that the stress fibers in astroglia may be contractile. Contractile stress fibers would enable astroglia to exert tension on the matrix surrounding them, thus facilitating rapid changes in cell shape.

Polymerization of actin induced by actin-binding fragments of caldesmon

Our earlier studies revealed that caldesmon causes assembly of G-actin into polymers morphologically indistinguishable from those formed in the presence of salt (Gałazkiewicz, B., Belagyi, J. and Dabrowska, R. (1989) Eur. J. Biochem. 181, 607-614). In this work we have investigated the effect of actin-binding fragments of caldesmon on actin polymerization process followed by measurements of the changes in fluorescence of pyrenyl conjugated with G-actin and ATP hydrolysis. The results indicate that C-terminal 34 kDa fragment of caldesmon containing two actin-binding sites and 19 kDa containing high-affinity binding site have similar capability to polymerize actin to that of intact molecule. Binding of each of these fragments to G-actin causes bypassing of nucleation phase. The 11.5 kDa fragment comprising low affinity actin-binding site has much lower potency to polymerize actin. Conformation of actin monomers in filaments formed upon 19 kDa fragment and that formed upon 11.5 kDa fragment differs. The former fragment seems to resemble more conformation of monomers in filaments formed upon intact caldesmon than the latter one.

Phosphorylation of aorta caldesmon by endogenous proteolytic fragments of protein kinase C

Endogenous caldesmon kinase activity in sheep aorta smooth muscle was purified and characterized. The enzyme was identified as a proteolytic fragment of protein kinase C by cross-reactivity with anti-protein kinase C antibodies, autophosphorylation, substrate specificity and the primary structure of the sites of phosphorylation on caldesmon. The enzyme phosphorylated aorta caldesmon both in native thin filaments and in the isolated state. Up to 2.9 mols of phosphate per mol of caldesmon were transferred. Prolonged incubation of caldesmon with the kinase resulted in phosphorylation of Ser-127, Ser-587, Ser-600, Ser-657, Ser-686, and Ser-726 (numbering corresponds to chicken gizzard caldesmon sequence). Ser-600 and Ser-587 were the major sites of phosphorylation containing more than 30% of phosphate transferred. Phosphorylation did not significantly affect the interaction of caldesmon with Ca(2+)-calmodulin. However, phosphorylation of both intact caldesmon and of its C-terminal fragment (658C), containing residues 658-756, significantly decreased their ability to inhibit acto-heavy meromyosin ATPase. This seems to be partially due to a decrease in the binding of caldesmon and 658C to actin-tropomyosin and partly due to an uncoupling of the binding-inhibition relationship.

Identification of functioning regulatory sites and a new myosin binding site in the C-terminal 288 amino acids of caldesmon expressed from a human clone

A partial clone of caldesmon, coding for the C-terminal 288 amino acids, was isolated from a human fetal liver cDNA library and sequenced. Expression of the clone in Escherichia coli produced a peptide called H1 (M(r) 32,549), which inhibited tropomyosin-enhanced actomyosin Mg(2+)-ATPase activity by 90% with half maximal inhibition at 0.03-0.04 mol H1 per mol actin. The inhibition could be reversed by Ca(2+)-calmodulin. H1 bound actin, Ca(2+)-calmodulin and tropomyosin and smooth muscle myosin with high affinities. This latter finding shows the presence of a second myosin-binding site in caldesmon. This was confirmed in thrombic digests of native sheep aorta and chicken gizzard caldesmon.

The functional effects of mutations Thr673-->Asp and Ser702-->Asp at the Pro-directed kinase phosphorylation sites in the C-terminus of chicken gizzard caldesmon

We expressed the C-terminal 99 amino acids of chicken gizzard caldesmon (658C) and two point mutants in which the preferred phosphorylation sites of MAP kinase and p34cdc2 kinase, Ser702 and Thr673 were altered to aspartic acid. The T673D mutant was indistinguishable from 658C but S702D was not phosphorylated by MAP kinase, was significantly less potent as an inhibitor of actin-tropomyosin activation of myosin MgATPase, and bound less actin-tropomyosin at low concentrations. Thus Ser702 is involved in the tropomyosin-dependent inhibitory mechanism of caldesmon, and its phosphorylation by MAP kinase or p34cdc2 kinase could modulate caldesmon function.

Electron microscopic images suggest both ends of caldesmon interact with actin filaments

An improved rotary shadowing technique enabled us to visualize chicken gizzard caldesmon (CaD) and its complexes with one or two covalently linked calmodulin (CaM) molecules by electron microscopy. Using a monoclonal antibody against an epitope in the N-terminal region of CaD (anti-N), we can now identify the end of the molecule that is involved in binding to another protein molecule. Thus in the 1:1 complex of CaD and CaM, the CaM molecule was almost always associated with the C-terminus of CaD, indicating preferential CaM-binding to the C-terminal region. We have also studied binding of CaD to filamentous actin (F-actin), using an EM technique that avoids spraying or freeze drying and thereby preserves the structure of F-actin. Only one end of CaD appeared to bind to F-actin, leaving the rest of the molecule projecting away from the filament. While the majority of anti-N bound at the free end of CaD, some antibody molecules were found on F-actin. These findings suggest that either end of CaD can bind to F-actin. Experiments using a monoclonal antibody against the C-terminus of CaD (anti-C) supported this idea. When the native thin filaments that contain endogenous CaD were incubated with anti-N, almost all the bound antibodies were found on the filaments, indicating that the N-terminal regions of CaD interact with actin, and that the binding affinity of the N-terminal region of CaD for actin is higher in vivo than that in vitro, either because the properties of CaD have been altered during purification, or because of the presence of some other component(s) associated with the native filaments.

Caldesmon

Recent research has led to an understanding of the in vitro properties of caldesmon, including the regulation of actomyosin ATPase activity, cross-linking between actin and myosin, enhancement of microfilament stability and stimulation of polymerization of actin. While it remains to be established whether caldesmon functions similarly in vivo, recent studies have suggested that smooth muscle caldesmon regulates the inhibition of vascular smooth muscle tone, and that non-muscle caldesmon plays roles in the regulation of cell motility and cytoskeletal organization in three biological activities: granule movement, hormone secretion and reorganization of microfilaments during mitosis.

Phosphorylation-contraction coupling in smooth muscle: role of caldesmon

In intact smooth muscle strips from chicken gizzard, carbachol elicited brief, phasic contractions which were associated with a very rapid, transient phosphorylation of the 20 kDa myosin light chains. Phosphorylation was not significantly different from basal levels after 30 s while force still amounted to 50% of the peak value. The rate of tension decline could be increased by addition of atropine, even at apparently basal phosphorylation levels suggesting a phosphorylation independent regulation. The force, at a given level of phosphorylation, could also be modulated by addition of the actin binding, putative regulatory protein, caldesmon. Caldesmon, inhibits phosphorylation dependent force in skinned fiber bundles of chicken gizzard without affecting myosin light chain phosphorylation. This suggests that caldesmon might modulate contraction in smooth muscle. Moreover our results suggest that caldesmon does not function to maintain passive tension.

A novel regulatory protein that affects the functions of caldesmon and myosin light chain kinase

A caldesmon (CaD)-binding protein of about 65 kDa (by SDS-PAGE) was purified from smooth muscle of chicken gizzard. The 65-kDa protein prevented the inhibitory effect of CaD on the ATP-dependent interaction between actin and myosin. Unlike the case with calmodulin (CaM), Ca2+ was not required for this effect. As reported in the preceding communication, myosin light chain kinase (MLCK), another well characterized protein that binds CaM, has CaD-like activity that modulates the interaction by binding to actin. The 65-kDa protein was also effective in relieving the modulation, while leaving unaffected the kinase activity that phosphorylates the light chain of smooth muscle myosin.

Caldesmon binds to smooth muscle myosin and myosin rod and crosslinks thick filaments to actin filaments

It is well established that caldesmon binds to actin (Kb = 10(7) - 10(-8) M-1) and to tropomyosin (Kb = 10(6) M-1) and that it is a potent inhibitor of actomyosin ATPase. Caldesmon can also bind tightly to myosin. We investigated the binding of smooth muscle and nonmuscle caldesmon isoforms (CDh and CDl respectively) to myosin using proteins from sheep aorta. Both caldesmon isoforms bind to myosin with indistinguishable affinity. The affinity is about 10(6) M-1 in low salt buffer, but is weakened by increasing [KCl] reaching 10(5) M-1 in 100 mM KCl. The stoichiometry of binding is about three caldesmon per myosin molecule. Stoichiometry and affinity are not dependent on whether myosin is phosphorylated nor on the presence of Mg2+ and ATP, provided the ionic strength is maintained constant. The caldesmon binding site of smooth muscle myosin is located in the S-2 region, consequently both HMM and myosin rod bind to caldesmon. Over a range of conditions myosin and myosin rod binding to caldesmon were indistinguishable. Skeletal muscle myosin has no caldesmon binding site. Smooth muscle myosin rods form side-polar filaments in low salt buffer in which the backbone packing of LMM into the filament shaft is clearly visible in negatively-stained electron microscopic images. Sometimes the S-2 portions can be seen 'frayed' from the filament shaft. When caldesmon is bound the filament shaft appears to be about 20% thicker and the frayed effect is dramatically increased; long filamentous 'whiskers' are often seen curving out from the filament shaft. Similar structures are observed with smooth muscle and with non-muscle caldesmon. Myosin also binds to caldesmon when it is incorporated into the thin filament; however, this interaction is qualitatively different. Measurements of smooth muscle HMM binding to native thin filaments in the presence of 3 mM MgATP shows there is a high affinity binding (Kb = 10(6) M-1) which is independent of [Ca2+] and of the level of myosin phosphorylation. The stoichiometry is one HMM molecule per actin monomer which is equivalent to up to 14 HMM bound at high affinity per caldesmon. Negatively stained electron microscopic images of the HMM.ADP.Pi-thin filament complex have failed to show any attachment of HMM to the thin filaments. When rod filaments are added to actin plus caldesmon or to native thin filaments the rod filaments are strongly associated with the actin filament bundles. The majority of rod filaments are lined up parallel and in close proximity to actin filaments.(ABSTRACT TRUNCATED AT 400 WORDS)

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Characterization of smooth muscle caldesmon as a microtubule-associated protein

We have previously shown that nonmuscle caldesmon copurified with brain microtubules binds to microtubules in vitro [Ishikawa et al.: FEBS Lett. 299:54-56, 1992]. To explore the role of caldesmon in the functions of microtubules, further characterization was performed using smooth muscle caldesmon, whose molecular structure and function have been best-characterized in all caldesmon species. Smooth muscle caldesmon bound to microtubules with a stoichiometry of five tubulin dimers to one molecule of caldesmon with the binding constant of 1.1 x 10(6) M-1. The binding of caldesmon to microtubules was inhibited in the presence of Ca2+ and calmodulin. Partial digestion of the caldesmon with alpha-chymotrypsin revealed that the binding site of the caldesmon for microtubules lay in the 34-kDa C-terminal domain. When the caldesmon was in the dimeric form in the absence of a reducing agent, the caldesmon cross-linked microtubules to form bundles. Further, the caldesmon potentiated the polymerization of tubulin, and inhibited the in vitro movement of microtubules on dynein. These results suggest that caldesmon may be involved in the regulation by Ca2+ of the functions of microtubules.

Determination of the phosphorylation sites of smooth muscle caldesmon by protein kinase C

Smooth muscle caldesmon was phosphorylated by protein kinase C up to 1.90 mol P/mol caldesmon. Phosphorylated caldesmon was completely digested by trypsin and the produced phosphopeptides were purified by C-8 and C-18 reverse phase chromatography. Four phosphopeptides were determined and two phosphoserines were identified. Both were localized in the C-terminal domain at serine-587 and serine-726. By following the time course of phosphorylation, serine-587 was found to be the preferred site. Effects of the phosphorylation of caldesmon by protein C on the inhibition of acto-H-meromyosin ATPase activity was also examined. While unphosphorylated caldesmon inhibited the ATPase activity by 60%, phosphorylated caldesmon hardly inhibited the ATPase activity. Therefore, it was concluded that the phosphorylation at serine-726 and serine-587 reverses the inhibitory activity of caldesmon.

Spectrofluorimetric studies on C-terminal 34 kDa fragment of caldesmon

Analysis of the tryptophan fluorescence emission spectra of caldesmon and its 34 kDa C-terminal fragment indicates that all tryptophan residues are located on the surface of the molecule, accessible to solvent. All three tryptophan residues of the 34 kDa fragment and four of the five tryptophan residues of intact protein are accessible to free water, whereas one located in the N-terminal region of molecule is accessible only to bound water molecules. The temperature dependence of the fluorescence parameters indicates higher thermal stability of the 34 kDa fragment than the whole caldesmon molecule. The interaction of the 34 kDa fragment of caldesmon (like that of the intact molecule) with calmodulin is accompanied by a blue shift of the fluorescence emission maximum and an increase in the relative quantum yield. Computer-calculated binding constants show that the binding of calmodulin to the 34 kDa fragment (K = 2.5 x 10(5) M-1) is of two orders of magnitude weaker than that to intact caldesmon (K = 1.4 x 10(7) M-1). The interaction with tropomyosin results in a blue shift of the spectrum of the 34 kDa fragment, yet there is no effect on the spectrum of intact caldesmon. Binding constants of tropomyosin to caldesmon (K = 3.8 x 10(5) M-1) and its 34 kDa fragment (K = 2.3 x 10(5) M-1) are similar. Binding of calmodulin to caldesmon and to the 34 kDa fragment affects their interaction with tropomyosin.

Electron microscopic studies of chicken gizzard caldesmon and its complex with calmodulin

Caldesmon samples mounted on a stage rotating about a horizontal axis were shadowed keeping the shadow angle at about 3 degrees. This technique minimizes background metal deposits compared with the conventional method. The identity of caldesmon was confirmed by comparing the images of caldesmon alone with those of the caldesmon-calmodulin complex. In these samples the caldesmon molecules appeared to be elongated; most were between 30 and 80 nm in length. The maximum length was in good agreement with the earlier estimate of 74 nm based on hydrodynamic studies. Our observations also suggested the presence of a rather rigid 30-40 nm stretch in the middle of the caldesmon molecule, which was always visible under rotary shadowing, and a flexible structure of about 20 nm in length at each end of the molecule, which may or may not be visible depending on their orientation on the mica surface. In the samples of caldesmon crosslinked with calmodulin, we noticed the existence of complexes containing two calmodulin molecules per caldesmon molecule, separated by a distance of 60 nm, consistent with the suggestion that each end of caldesmon can interact with calmodulin.

Epitope mapping of monoclonal antibodies against caldesmon and their effects on the binding of caldesmon to Ca++/calmodulin and to actin or actin-tropomyosin filaments

The effects of monoclonal anti-caldesmon antibodies, C2, C9, C18, C21, and C23, on the binding of caldesmon to F-actin/F-actin-tropomyosin filaments and to Ca++/calmodulin were examined in an in vitro reconstitution system. In addition, the antibody epitopes were mapped by Western blot analysis of NTCB (2-nitro-5-thiocyanobenzoic acid) and CNBr (cyanogen bromide) fragments of caldesmon. Both C9 and C18 recognize an amino terminal fragment composed of amino acid residues 19 to 153. The C23 epitope lies within a fragment ranging from residues 230 to 386. Included in this region is a 13-residue repeat sequence. Interestingly this repetitive sequence shares sequence similarity with a sequence found in nuclear lamin A, a protein which is also recognized by C23 antibody. Therefore, it is likely that the C23 epitope corresponds to this 13-residue repeat sequence. A carboxyl-terminal 10K fragment contains the epitopes for antibodies C2 and C21. Among these antibodies, only C21 drastically inhibits the binding of caldesmon to F-actin/F-actin-tropomyosin filaments and to Ca++/calmodulin. When the molar ratio of monoclonal antibody C21 to caldesmon reached 1.0, a maximal inhibition (90%) on the binding of caldesmon to F-actin filaments was observed. However, it required double amounts of C21 antibody to exhibit a maximal inhibition of 70% on the binding of caldesmon to F-actin-tropomyosin filaments. These results suggest that the presence of tropomyosin in F-actin enhances caldesmon's binding. Furthermore, C21 antibody also effectively inhibits the caldesmon binding to Ca++/calmodulin. The kinetics of C21 inhibition on caldesmon's binding to Ca++/calmodulin is very similar to the inhibition obtained by preincubation of caldesmon with free Ca++/calmodulin. This result suggests that there is only one Ca++/calmodulin binding domain on caldesmon and this domain appears to be very close to the C21 epitope. Apparently, the Ca++/calmodulin-binding domain and the actin-binding domain are very close to each other and may interfere with each other. In an accompanying paper, we have further demonstrated that microinjection of C21 antibody into living chicken embryo fibroblasts inhibit intracellular granule movement, suggesting an in vivo interference with the functional domains [Hegmann et al., 1991: Cell Motil. Cytoskeleton 20:109-120].

Inhibition of intracellular granule movement by microinjection of monoclonal antibodies against caldesmon

Monoclonal antibodies, C2, C9, C18, and C21, against chicken gizzard caldesmon (called high molecular weight isoform) were shown to crossreact with a low molecular weight isoform of caldesmon in chicken embryo fibroblasts (CEF). These antibodies were used in a microinjection study to investigate the in vivo function of caldesmon in nonmuscle cell motility. Injected cells did not appear to change their morphology significantly; the cells displayed a flat appearance and were able to ruffle and locomote normally. However, in the C21 injected cells, saltatory movements of granules and organelles appeared to be greatly inhibited. This inhibition of granule movement was reversible, so that by 3 hr after injection, granules in injected cells had already recovered to normal speed. The inhibition of granule movement in cells injected with C2, C9, or C18 antibody, or with C21 antibody preabsorbed with caldesmon, were not significantly different from that in uninjected cells. In a previous epitope study, we demonstrated that, of the antibodies used in this study, only C21 antibody was able to compete with the binding of caldesmon to Ca++/calmodulin and to F-actin, although both C21 and C2 antibodies recognized the same carboxyl-terminal 10K fragment of gizzard caldesmon [Lin et al., 1991: Cell Motil. Cytoskeleton 20:95-108]. The caldesmon distribution in C21 injected cells changed from stress-fiber localization to a more diffuse appearance, when the injection was performed at 10-30 mg/ml of C21 antibody. We have previously shown that a monoclonal anti-tropomyosin antibody exhibited motility-dependent recognition of an epitope, and that microinjection of this antibody specifically inhibited intracellular granule movements of CEF cells [Hegmann et al., 1989: J. Cell Biol. 109:1141-1152]. Therefore, it is likely that tropomyosin and caldesmon may both function in intracellular granule movement by regulating the contractile system in response to [Ca++] change inside nonmuscle cells.

Structural study of gizzard caldesmon and its interaction with actin. Binding involves residues of actin also recognised by myosin subfragment 1

The interaction between actin and caldesmon that is associated with the inhibition of actomyosin ATPase activity in smooth muscle has been studied using 1H-NMR spectroscopy. Binding studies using the intact molecules were complemented by the use of thrombic cleavage fragments of both turkey and chicken gizzard caldesmon as well as defined peptides of actin, in order to investigate the conformational properties of caldesmon and to localise regions of the primary structures that participate in protein-protein contacts. The binding of caldesmon is shown to involve distinct segments on the N-terminal region (residues 1-44) of actin, as previously observed for the inhibitory component of the thin filament of striated muscle, troponin I [Levine et al. (1988) Eur. J. Biochem. 153, 389-397]. The comparable structural properties of these tissue-specific inhibitors of actomyosin ATPase and the similarities in their mode of interaction at the N-terminal region of actin suggest common aspects to the structural mechanism for thin-filament regulation in smooth and striated muscle. Unlike the inhibitory interaction of troponin I, however, the binding of caldesmon to the N-terminal region of actin directly involves groups within residues 20-41 of actin that are also recognised by myosin subfragment 1. The complementary segment of caldesmon has been localised to a 15-kDa thrombic fragment (residues 483-578) derived from the N-terminal portion of a 35-kDa proteolytic cleavage product from the C-terminal of caldesmon whose interaction with actin is modulated by calmodulin. The results are discussed in relation to the calcium-mediated mechanism for thin-filament regulation in smooth and striated muscle.

The functional properties of full length and mutant chicken gizzard smooth muscle caldesmon expressed in Escherichia coli

Wild type chicken gizzard caldesmon (756 amino acids) was expressed in a T7 RNA polymerase-based bacterial expression system at a yield of 1 mg pure caldesmon per litre bacterial culture. A mutant composed of amino acids 1-578 was also constructed and expressed. The wild type and mutant caldesmon were purified and compared with native chicken gizzard caldesmon. Native and wild type expressed caldesmon were indistinguishable in assays for inhibition of actin-tropomyosin activation of myosin ATPase, reversal of inhibition by Ca2(+)-calmodulin and binding to actin, actin-tropomyosin, Ca2(+)-calmodulin, tropomyosin and myosin. The mutant missing the C-terminal 178 amino acids had no inhibitory effect and did not bind to actin or Ca2(+)-calmodulin. It bound to tropomyosin with a 5-fold reduced affinity and to myosin with a greater than 10-fold reduced affinity.

Caldesmon, calmodulin and tropomyosin interactions

Binary complex interactions between caldesmon and tropomyosin, and calmodulin and tropomyosin, and ternary complex interaction involving the three proteins were studied using viscosity, electron microscopy, fluorescence and affinity chromatography techniques. In 10 mM NaCl, caldesmon decreased the viscosity of chicken gizzard tropomyosin by 7-8 fold with a concomitant increase in turbidity (A330nm). Electron micrographs showed spindle-shaped particles in the tropomyosin-caldesmon samples. These results suggest side-by-side aggregation of tropomyosin polymers induced by caldesmon. Binding studies in 10 mM NaCl between caldesmon and chicken gizzard tropomyosin labelled with the fluorescent probe N-(1-anilinonaphthyl-4)maleimide (ANM) gave association constants from 5.3.10(6) to 7.9.10(6) M-1 and stoichiometry from 1.0 to 1.4 tropomyosin per caldesmon. Similar binding was observed for rabbit cardiac tropomyosin and caldesmon. Removal of 18 and 11 residues from the COOH ends of the gizzard and cardiac tropomyosin by carboxypeptidase A, respectively, had no significant effect on their binding to caldesmon. In the presence of Ca2+, chicken gizzard tropomyosin bound to a calmodulin-Sepharose-4B column and was eluted with a salt concentration of 140 mM. This interaction was weakened in the absence of Ca2+, and the bound tropomyosin was eluted by 65 mM KCl. ANM-labelled tropomyosin bound calmodulin in the presence of Ca2+ with a binding constant of 3.5.10(6) M-1 and a binding stoichiometry of 1 to 1.4 tropomyosin per calmodulin. In 10 mM NaCl, calmodulin reduced the specific viscosity of chicken gizzard tropomyosin in the presence of Ca2+ by 5 fold, while a 1.5-fold reduction in viscosity was observed in the absence of Ca2+. In either case, no significant increase in turbidity was observed suggesting that calmodulin reduced head-to-tail polymerization of tropomyosin. The interaction of caldesmon with the calmodulin-ANM-tropomyosin complex in the presence and absence of Ca2+ was also examined. The result is consistent with a model that in the absence of Ca2+, calmodulin binds weakly to either caldesmon or tropomyosin and has little effect on the tropomyosin-caldesmon interaction; whereas, Ca2(+)-calmodulin interacts with caldesmon and reduces its affinity to tropomyosin.

Calcium regulated thin filaments from molluscan catch muscles contain a caldesmon-like regulatory protein

The thin filaments of the anterior byssus retractor muscle of the edible mussel Mytilus and the transluscent and opaque adductors of the oyster Crassostrea have been isolated and their properties investigated. We find that the thin filaments from all three muscles can activate skeletal muscle myosin ATPase in the presence of calcium but that the activity is inhibited in its absence. The filaments contain a protein which interacts with antibodies to vertebrate smooth muscle caldesmon on immunoblots. The antibodies relieve the inhibition of the thin-filament-activated myosin MgATPase. They can also bundle the thin filaments. We conclude that a caldesmon-like protein is present in molluscan muscle. As in the vertebrate smooth muscle, it could act as part of a control mechanism in addition to the myosin regulatory system. Vertebrate smooth muscle caldesmon can crosslink actin and myosin and it has been suggested that it may in this way contribute to the latch state. A similar interaction may be involved in the catch mechanism in molluscan muscle.

Mitosis-specific phosphorylation causes 83K non-muscle caldesmon to dissociate from microfilaments

At mitosis in eukaryotic cells there are profound changes of shape and structure whose causes are almost entirely obscure. What is known is that there are changes in the organization of microfilaments, including the disassembly of microfilament bundles during prophases and the accompanying rounding-up of cultured cells; the formation of transient contractile rings during cytokinesis; and, subsequently, the reassembly of microfilament bundles and the respreading of the two daughter cells. As an initial step towards the biochemical understanding of these events, in which the disassembly and reassembly of microfilaments appear to play an important part, we searched for alterations of the molecular constitution of microfilaments during mitosis. We found that non-muscle caldesmon, a protein with a relative molecular mass (Mr) of 83,000 (83K) which binds to actin and calmodulin, is dissociated from microfilaments during mitosis, apparently as a consequence of phosphorylation. This process may contribute to the changes of shape and structure of cells in mitosis, as caldesmon inhibits actomyosin ATPase.

Smooth muscle myosin as a calmodulin binding protein. Affinity increase on filament assembly

Smooth muscle myosin is normally copurified with myosin light chain kinase (MLCKase) and calmodulin (CM). We have now established the binding affinities and stoichiometries of these two components with respect to monomeric and filamentous myosin. The relative amounts of CM and MLCKase in fresh synthetic myosin filaments were approximately stoichiometrical but for both in a molar ratio to myosin of about 1 to 30 or less. A 10(7) dilution of filaments did not result in any significant decrease in the amount of endogenous MLCKase and CM except in the absence of Ca2+ when the CM content was reduced around five-fold. Binding assays were performed with myosin depleted of CM and MLCKase by passage over melittin- and CM-affinity columns, arranged in tandem. For binding to myosin preassembled into filaments three classes of CM binding sites could be demonstrated. (1) A high affinity binding characterized by a dissociation constant of 20-30 nM and a rather low binding stoichiometry of below 1 to 500. (2) An intermediate affinity, characterized by a dissociation constant of 1.2 microM and 1 to 100 binding stoichiometry. (3) A low affinity with a Kd greater than 10 microM and with an approximate 1 to 1 binding ratio relative to myosin. If CM was made available during filament assembly the high affinity binding predominated, with a stoichiometry in the presence of Ca2+ of about 1 to 50. The binding affinity but not the stoichiometry was reduced several fold by the removal of Ca, excluding a non-specific trapping of CM within the filament architecture. Collectively, these data demonstrate an independent and specific association of MLCKase and CM with myosin, that is strengthened by filament assembly.

Phosphorylation of high-Mr caldesmon by protein kinase C modulates the regulatory function of this protein on the interaction between actin and myosin

High-Mr caldesmon, which is involved in smooth muscle contraction, was phosphorylated by protein kinase C. By chymotryptic digestion, actin- and calmodulin-binding assays and immunoprecipitation with the antibody to the C-terminal 35-kDa fragment, we have identified that all phosphate groups are incorporated exclusively into this fragment, which is the functional domain for binding actin and calmodulin. Phosphorylation of high-Mr caldesmon and its C-terminal 35-kDa fragment reduced their binding abilities to both F-actin and calmodulin. Further, their inhibitory effects on the actin-activated ATPase activity of gizzard myosin were also reversed in proportion to the degree of phosphorylation. These results suggest that phosphorylation of high-Mr caldesmon by protein kinase C, which is restricted within the C-terminal 35-kDa domain, results in the modulation of its activity in the smooth muscle actin--myosin interaction.